Sapphire is a single-crystalline aluminum oxide transparent to the visible light. Natural sapphire may include various impurities according to the environment, and thus the energy band gap of the single-crystalline aluminum oxide is influenced by the impurities. For this reason, sapphire may have peculiar colors, such as blue, green, red, yellow, etc., depending on the species of the impurities, and can be used as gemstones. Generally, the colored sapphire is produced in nature in large quantities. Since the amount and kind of the impurities included in sapphire are predetermined, the ability to improve the value of sapphire is limited even when the sapphire is post-treated in order to use the sapphire as a gemstone. Unlike diamond, the colored sapphire has a several times higher value than the transparent sapphire. Therefore, the present invention provides a method—enabling sapphire to exhibit various colors by doping various metallic impurities into the transparent sapphire.
As conventional gemstone coloring methods, a radiation method, a surface diffusion method, and an ion implantation method are used.
First, the radiation method employs a principle in which high-energy particles, such as α-rays or γ-rays, are applied on the surface of a gemstone, so that lattice defects are induced, thereby enabling the jewel to exhibit colors. That is, when sapphire is irradiated with radioactive rays, lattice defects are induced, and the lattice defects act as color centers. From this principle, sapphire can exhibit various colors according to the kind of the radiation and the intensity of the radiant energy. However, the radiation method, in which lattice defects are induced temporarily, is problematic in that, since sapphire is stabilized in combination with oxygen, even when the sapphire is left in the atmosphere for a certain period or is heated slightly, the sapphire is combined with oxygen, so that lattice defects disappear, with the result that the color of the sapphire returns to its original color. Further, this radiation method is problematic in that the radioactive rays applied to sapphire are continuously emitted from the sapphire, and it takes 10 years or more to make the sapphire, which is contaminated with radio-active radiations.
Meanwhile, the surface diffusion method is a method of diffusing impurities into the sapphire by coating the materials on the surface of the sapphire and then heat-treating. Compared to the above the radiation method, this surface diffusion method can solve the problem in which the color of the sapphire returns to its original color and the problem in which radioactive rays are emitted from the sapphire, but is problematic in that a long period and high-temperature treatment conditions are required in order for the impurities to diffuse deeper into the sapphire, and thus the production cost of the sapphire is relatively high.
Further, the ion implantation method is a method of implanting ions into a sapphire by accelerating the ionized elements in a vacuum and is advantageous in that its energy band gap is changed after a post-implantation annealing depending on the kind of ion, and thus various colors are exhibited. However, the ion implantation method is problematic in that the kinds of ions and the chemical reactivity must be considered. Further, this ion implantation method is problematic in that, when the ion energy is excessively high at the time of implanting ions into a sapphire, the surface of the sapphire is damaged by radiation, and when energy is excessively weak, ion implantation occurs only on the surface of the sapphire, so that the sapphire using this ion implantation method is not different from the sapphire using the above surface diffusion method.
Meanwhile, the coloring effects of sapphire due to the addition of elements are different from those of other gemstones. For example, since diamond is composed of carbon, when nitrogen or boron is added to the diamond, the diamond exhibits yellow and blue colors. In contrast, sapphire does not exhibit peculiar colors even when nitrogen or boron, which are non-metallic elements, are added thereto, but can exhibit peculiar colors when metallic elements are added thereto. The reason for this is that the energy band gap (about 9.0 eV) of sapphire is far larger than that (about 5.5 eV) of diamond.
However, not all metallic elements addition exhibits valuable colors by the sapphire when doped by a post-implantation annealing. For example, it was reported in the paper “Nuclear Instrument and Methods in Physics Research b 59 (1991)1173-1176”, written by C. Marques, et al., that when gold (Au) is added to sapphire, the sapphire exhibits a dark black color. However, when heat treatment is conducted, sapphire and gold are not reactive with each other and thus cannot be bonded with aluminum and oxygen, and are thus isolated from the bulk sapphire, resulting in no color change. Therefore, the reason why the sapphire exhibits a dark black color is that the sapphire is only damaged by the energy irradiated in the sapphire.
Meanwhile, it was reported in the paper “Nuclear Instrument and Methods in Physics Research b 218 (2004)139-144”, written by C. Saito, et al., that cobalt ions are added to sapphire, and then the sapphire is heat-treated at 800° C. and 1000° C., thus obtaining sapphire exhibiting a green color or a light blue color. Here, since the ion energy used is 20 keV, which is low, the depth of the ions implantation into the sapphire is very shallow. However, due to the heat treatment, the ions implantation into the sapphire only to a very shallow depth, exhibiting the sapphire a green color or a light blue color.
Further, Alves, et al. reported in the paper “Nuclear Instrument and Methods in Physics Research b 207 (2003) 55-62” the phenomena occurring in heat treatment after the addition of titanium and cobalt ions to sapphire. As a result, aluminum in sapphire is substituted with titanium and cobalt, and the sapphire remains stable up to a temperature of 1000° C. They also reported that, when heat treatment is conducted under a reducing atmosphere or when a large amount of ions is added to sapphire, the added Ti or Co ions are bonded with each other, and thus they exist in a metallic state. Therefore, the amount of ions added to the sapphire and the heat treatment atmosphere are important factors in the ion implantation method for the sapphire coloration.
Therefore, the present inventors have conducted research in order to manufacture sapphire which is not damaged by radiation and in which colors are uniformly distributed. As a result, they have found that when sapphire is irradiated with a predetermined amount of metal ions having a predetermined energy level, the metal ions are implanted into the sapphire, and when the sapphire is heated in an oxidizing atmosphere, the radiation damage caused by the ion implantation is removed, and simultaneously the metal ions are uniformly diffused into the sapphire. Based on these findings, the present invention was completed.